Prior art of controlling a synchronous motor without detecting an electrical angular position is disclosed in, for example, JP-A No. 2002-78392 (hereinafter referred to as Japanese Patent Document 1) which relates to a method for estimating the position of a magnetic pole within the synchronous motor.
JP-A No. 2001-95215 (hereinafter referred to as Japanese Patent Document 2) discloses effects of magnetic saturation occurred locally in a stator of a permanent magnet synchronous motor (PM motor).
The method according to Japanese Patent Document 1 comprises the steps of applying voltage pulses to the synchronous motor in the directions of two axes perpendicular to each other, measuring the amplitudes of the current pulses generated in the directions of both axes, and estimating the magnetic pole position, based on the measurements. In this method, by applying approximation to a relation between the generated currents and the estimated magnetic pole position, compatibility between the number of times of applying the voltage pulses and the accuracy of the estimation is achieved.
However, with regard to the change in ripple components of the above currents due to magnetic saturation, Japanese Patent Document 1 makes an assumption as will be described below. FIGS. 14A to 14C show a relation between permanent magnet flux φm and the generated current Idc, which is assumed by Japanese Patent Document 1. FIG. 14A shows a dc axis and the direction of permanent magnet flux φm inside the motor; FIG. 14B shows a relation between the current Idc and the primary flux φId; and FIG. 14C is a schematic diagram showing the waveform of the current Idc. Here, it is supposed that the dc axis along which the voltage pulses are applied is aligned with the direction of permanent magnet flux φm inside the motor as shown in FIG. 14A. When the direction of the current Idc is aligned with the direction of permanent magnet flux φm, the current Idc generates flux in the same direction as the direction of permanent magnet flux φm, which acts to accelerate magnetic saturation of the motor core. At this time, inductance Lds0 is smaller than inductance Ld0 that would be measured if the direction of the current Idc is opposite to the direction of permanent magnet flux φm, and the current Idc changes as shown in FIG. 14C.
However, in the case that magnetic saturation occurred locally in the stator of the PM motor has an effect as described in Japanese Patent Document 2, the assumption by Japanese Patent Document 1 is not always true, depending on the magnitude of the current Idc, and there is a possibility of a major error in estimating the magnetic pole position. The effect of the local magnetic saturation depends on the PM motor structure and can be reduced relatively by increasing the current Idc, but may be restricted by a controller that drives the motor.